The utilization of automatic dispensers to dispense chemicals used in cleaning processes is well known in the art. The automatic dispensers may generally be placed into two broad categories based upon their method of controlling the amount of chemical dispensed; (1) time controlled dispensers, and (2) conductivity measurement dispensers.
Time controlled dispensers can only dispense solutions of known and/or constant concentration for if the concentration is unknown and variable different amounts of chemical will be dispensed during each cycle.
One example of a widely utilized method of dispensing a solution used in cleaning processes wherein the concentration of the solution dispensed will be unknown and variable is described in U.S. Pat. No. 4,063,663 issued to Larson et al, which is expressly incorporated by reference herein. Larson discloses a dispenser wherein water is sprayed onto and dissolves the downward facing surface of a granular detergent for use in a washing machine.
In attempts to control the quantity of chemical dispensed when the concentration of the solution is unknown or variable the relationship between solution concentration and temperature and conductivity of the solution can be utilized.
For example, the effect of concentration and temperature upon the conductivity of sodium hydroxide solutions is presented in Table 1 and Graph 1 respectively. Actual test data obtained from the dispensing system and the chemical dispensed will result in a generally observable and reproducible relationship between these three variables for that system.
Prior art devices control the quantity of chemical dispensed by measuring the conductivity of either (i) the wash water, or (ii) the concentrated chemical solution held in a reservoir with concentrated chemical solution being dispensed into the measured reservoir when the conductivity of the measured solution falls below a predetermined set value.
It is preferable to measure the conductivity of the concentrated chemical solution because: (i) the wash water contains contaminants such as soil which can affect the conductivity of the wash water, (ii) there can be a large time lag between dispensing of the concentrated chemical solution and sensing of the change in conductivity of the wash water made by the additional chemical, and (iii) automatic dispensing devices are generally sold separately from the washing machine with which they are to be used and conductivity measurement of the wash water requires the implantation of electrodes into the washing machine requiring additional labor, added expense, and increasing the chance of failure.
Measurement of the conductivity of concentrated chemical solution, used in the cleaning process, which is contained in a separate reservoir avoids the problems listed above but requires a separate reservoir to maintain concentrated chemical solution, increases the health hazards associated with the dispensing of chemicals used in the cleansing process as concentrated chemical solution is constantly present and may be spilled or splashed onto an operator, and requires an additional mechanism for time controlled dispensing of the concentrated chemical solution from the reservoir into the washing machine.
Accordingly, a need exists for a compact dispenser which can dispense a desired quantity of a chemical in an aqueous chemical solution of an unknown and/or variable concentration in a safe, simple and accurate manner.